CR Image Acquisition
By Professor Stelmark
Unlike a film radiograph that is made up of minute deposits of black metallic
silver, a digital image is recorded as a matrix or combination of rows and columns
(array) of small, usually square, “picture elements” called pixels. Each pixel is
recorded as a single numerical value, which is represented as a single brightness
level on a display monitor. The location of the pixel within the image matrix
corresponds to an area within the patient or volume of tissue.
Assume that pixel values from 0 to 2048 are used to represent the full range of
digital image densities or brightness levels. A high pixel value could represent a
volume of tissue that attenuated fewer x-ray photons and is displayed as a
decreased brightness level or increased density. Therefore a low pixel value
represents a volume of tissue that attenuates more x-ray photons and is
displayed as increased brightness or decreased density.
Acquisition
During image acquisition the computer creates a histogram A histogram is a
graphic representation of a data set. This graph represents the number of
digital pixel values versus the relative prevalence of those values in the
image. The x-axis represents the amount of exposure and the y-axis the
incidence of pixels for each exposure level. The computer then analyzes the
histogram using processing algorithms and compares it to a preestablished
histogram specific to the anatomic part being imaged. This process is called
histogram analysis.
The computer software has histogram models for all menu choices. These
stored histogram models have values of interest (VOI) and determine what
section of the histogram data set should be included in the displayed image.
During this process of “recognition” the computer identifies the exposure field
and the edges of the image, and all exposure data outside this field are
excluded from the histogram. Ideally, all four edges of a collimated field are
recognized. If at least three edges are not identified, then all data, including
raw exposure or scatter outside the field, may be included in the histogram,
resulting in a histogram analysis error.
Histogram analysis is also employed to maintain consistent image
brightness despite overexposure or underexposure of the IR. This
procedure is known as automatic rescaling. The computer rescales the
image based on the comparison of the histograms, which is actually a
process of mapping the grayscale to the value of interest VOI to present
a specific display of brightness.
Part Selection
Once the patient has been positioned and the plate has been exposed, you must
select the examination or body part from the menu choices on your workstation.
For example, if you are performing a skull examination, select “skull” from the
workstation menu. Selecting the proper body part and position is important for the
proper conversion to take place. Image recognition is accomplished through
complex mathematical computer algorithms, and if the improper part and/or
position is selected, the computer will misinterpret the image. For example, if a
knee examination is to be performed and the examination selected is for skull, the
computer will interpret the exposure for the skull, resulting in improper density and
contrast and inconsistent image graininess .
It is not acceptable to select a body part or position different from that being
performed simply because it looks better. If the proper examination/part selection
results in a suboptimal image, then service personnel should be notified of the
problem to correct it as soon as possible. Improper menu selections may lead to
overexposure of the patient and/or repeats.
Kilovoltage Peak Selection
Kilovoltage peak (kVp), milliamperage seconds (mAs), and distance are chosen in
exactly the same manner as for conventional film/screen radiography. kVp must be
chosen for penetration and the type and amount of contrast desired. In the early
days of CR, kVp minimum values were set at about 70kVp. This is no longer
necessary. kVp values now range from around 45 to 120. It is not recommended
that kVp values less than 45 or greater than 120 be used because those values
may be inconsistent and produce too little or too much excitation of the phosphors.
However, exposures outside that range are widely used and will depend on the
quality desired. Remember, the process of attenuation of the x-ray beam is exactly
the same as in conventional film/screen radiography. It takes the same kVp to
penetrate the abdomen with CR systems as it did with a film/screen system. It is
vital that the proper balance between patient dose and image contrast be achieved.
Milliamperage Seconds Selection
The mAs is selected according to the number of photons needed for a
particular part. If there are too few photons, no matter what level of kVp is
chosen, the result will be a lack of sufficient phosphor stimulation. When
insufficient light is produced, the image is grainy, a condition known as
quantum mottle or quantum noise.
Imaging Plate Selection
Two important factors should be considered when selecting the CR imaging
cassette: type and size. Most manufacturers produce two types of imaging
plates: standard and high resolution. Cassettes should be marked on the
outside to indicate high resolution imaging plates. Typically, high resolution
imaging plates are limited to size range and are most often used for
extremities, mammography, and other examinations requiring increased detail.
In conventional film/screen radiography, we are taught to select a cassette
appropriate to the size of the body part being imaged. CR cassette selection is
the same but even more critical. CR digital images are displayed in a matrix of
pixels, and the pixel size is an important factor in determining the resolution of
the displayed image. The CR reader scans the imaging plate at a relatively
constant frequency, about 2000 × 2000 pixels. Using the smallest imaging plate
possible for each examination results in the highest sampling rate. When the
smallest possible imaging plate is selected, a corresponding matrix is used by
the computer algorithm to process the image. A 2000 × 2000 matrix on an 8′′ ×
10′′ cassette results in much smaller pixel size, thereby increasing resolution. If,
for example, a hand was imaged on a 14′′ × 17′′ cassette , the entire cassette is
read according to a 14′′ × 17′′ matrix size with much larger pixels so that the
resultant image is very large.
Grid Selection
Digital images are displayed in tiny rows of picture elements or pixels. Grid
lines that are projected onto the imaging plate when using a stationary grid can
interfere with the image. This results in a wavy artifact known as a moiré
pattern that occurs because the grid lines and the scanning laser are parallel
The oscillating motion of a moving grid, or Bucky, blurs the grid lines and
eliminates the interference. Because of the ability of CR imaging plates to
record a very high number of x-ray photons, the use of a grid is much more
critical than in film/screen radiography. Appropriate selection of stationary grids
reduces this interference as well. Grid selection factors are frequency, ratio,
focus, and size.
Frequency
Grid frequency refers to the number of grid lines per centimeter or lines per
inch. The higher the frequency or the more lines per inch, the finer the grid
lines in the image and the less they interfere with the image. Typical grid
frequency is between 80 and 152lines/in. Some manufacturers recommend
no fewer than 103lines/in and strongly suggest grid frequencies greater than
150. The higher the frequency, the less positioning latitude is available,
increasing the risk for grid cutoff errors, especially in mobile radiography. In
addition, the closer the grid frequency is to the laser scanning frequency, the
greater likelihood of frequency harmonics or matching and the more likely the
risk of moiré effects.
Ratio
The relationship between the height of the lead strips and the space between
the lead strips is known as grid ratio. The higher the ratio, the more scatter
radiation is absorbed. However, the higher the ratio, the more critical the
positioning is, so high grid ratio is not a good choice for mobile radiography. A
grid ratio of 6:1 would be proper for mobile radiography, whereas a 12:1 grid
ratio would be appropriate for departmental grids that are more stable and
less likely to be mispositioned, causing grid cutoff errors.
Size
The physical size of the grid matters in CR examinations. The smaller the
cassette being used, the higher the sampling rate. When using cassettes
that are 10′′ × 12′′ or smaller, it is important to select a high frequency grid
to eliminate scatter that will interfere with quality image interpretation by
the computer algorithm. Remember that the CR imaging plate is able to
record a wider range of exposure, including scatter.
Collimation
When exposing a patient, the larger the volume of tissue being irradiated and the
greater the kVp used, the more likely it is that Compton interactions, or scatter, will
be produced. Whereas the use of a grid absorbs the scatter that exits the patient
and affects latent image formation, properly used collimation reduces the area of
irradiation and the volume of tissue in which scatter can be created. Collimation is
the reduction of the area of beam that reaches the patient through the use of two
pairs of lead shutters encased in a housing attached to the x-ray tube. Collimation
results in increased contrast as a result of the reduction of scatter as fog and
reduces the amount of grid cleanup necessary for increased resolution.
Use of Lead Masks
Use of lead masks/blocker for multiple images on a single IR is recommended
when CR is used .This recommendation is due to the hypersensitivity of image
plate phosphors to lower-energy scatter radiation; even small amounts may
affect the image.
(Note: Some manufacturers recommend that only one image be centered and
placed per IP. Check with your department to find out whether multiple images
can be placed on a single IP.)
Through postexposure image manipulation known as shuttering, a black
background can be added around the original collimation edges, virtually
eliminating the distracting white or clear areas. However, this technique is not a
replacement for proper preexposure collimation. It is an image aesthetic only and
does not change the amount or angles of scatter. There is no substitute for
appropriate collimation because collimation reduces patient dose.
Side/Position Markers
If you have used CR image processing equipment, you already know
that it is very easy to mark images with left and right side markers or
other position or text markers after the exposure has been made.
However, we strongly advise that conventional lead markers be used
the same way they are used in film/screen systems. Marking the
patient examination at the time of exposure not only identifies the
patient’s side but also identifies the technologist performing the
examination. This is also an issue of legality. If the examination is used
in a court case, the images that include the technologist’s markers
allow the possibility of technologist testimony and lend credibility to his
or her expertise.
Exposure Indicators
The amount of light given off by the imaging plate is a result of the radiation
exposure the plate has received. The light is converted into a signal that is used to
calculate the exposure indicator number. This number varies from one vendor to
another (The total signal is not a measure of the dose to the patient but indicates
how much radiation was absorbed by the plate, which gives only an idea of what the
patient received.) The base exposure indicator number for all systems designates
the middle of the detector operating range.
For the Fuji (Tokyo, Japan), Philips (Eindhoven, The Netherlands), and Konica
Minolta (Tokyo, Japan) systems, the exposure indicator is known as the S or
sensitivity number. It is the amount of luminescence emitted at 1mR at 80kVp and
has a value of 200. The higher the S number with these systems, the lower the
exposure. For example, an S number of 400 is half the exposure of an S number
of 200, and an S number of 100 is twice the exposure of an S number of 200. The
numbers have an inverse relationship to the amount of exposure so that each
change of 200 results in a change in exposure by a factor of 2.
Kodak (Rochester, NY) uses exposure index (EI) as the exposure indicator. A 1mR exposure at 80kVp combined with aluminum/copper filtration yields an EI
number of 2000. An EI number plus 300 (EI + 300) is equal to a doubling of
exposure, and an EI number of –300 is equal to halving the exposure. The
numbers for the Kodak system have a direct relationship to the amount of
exposure, so that each change of 300 results in change in exposure by a factor
of 2.
The term for exposure indicator in an Agfa (Mortsel, Belgium) system is the
logarithm of the median exposure (lgM). An exposure of 20μGy at 75kVp with
copper filtration yields a lgM number of 2.6. Each step of 0.3 above or below
2.6 equals an exposure factor of 2
Recommended Exposure Indices
Overexposure Underexposure
Adult:
Nongrid and
Grid
Distal
Extremities
Nongrid
Kodak
>2500
<1600 tabletop;
<1800 Bucky
1800–2100
2200–2400
Agfa
>2.9
<2.1
2.1–2.3
2.4–2.6
Fuji/Philips/
Konica Minolta
<100
>250 tabletop;
>400 Bucky
200–300
75–125
Kodak
Agfa
Fuji/Philips/Konica
2000
2.6
200
Relative sensitivity
+300 = 2x 2300
+0.3 = 2x 2.9
½ S = 2x
x = exposure
–300 = ½ x 1700
–0.3 = ½ x 2.3
2x S = ½ x 400
Sensitivity value
100
Low exposure index (high “S” number) indicates underexposure with “noisy”
undesirable image.